LiDAR (light detection and ranging) uses laser technology to make precise distance measurements over short or long distances. LiDAR units have found widespread application in both industry and the research community.
The predecessor technology to current LiDAR units were object detection systems that could sense the presence or absence of objects within the field of view of one or more light beams based on phase shift analysis of the reflect light beam. Examples of these kinds of object detection systems in the field of vehicle “blind spot” warning systems include U.S. Pat. Nos. 5,122,796, 5,418,359, 5,831,551, 6,150,956, and 6,377,167.
Current LiDAR units are typically scanning-type units that emit beams of light in rapid succession, scanning across the angular range of the unit in a fan-like pattern. Using a time of flight calculation applied to any reflections received, instead of just a phase shift analysis, the LiDAR unit can obtain range measurements and intensity values along the singular angular dimension of the scanned beam. LiDAR units typically create the scanning beam by reflecting a pulsed source of laser light from a rotating mirror. The mirror also reflects any incoming reflections to the receiving optics and detector(s).
Single-axis-scan LiDAR units will typically use a polygonal mirror and a pulsed laser source to emit a sequence of light pulses at varying angles throughout the linear field of view. Return signals are measured by a bandpass photoreceptor that detects the wavelength of light emitted by the laser. The field of view of the photoreceptor covers the entire one-dimensional scan area of the laser. Thus, each subsequent emitted pulse of laser light must occur only after the reflected signal has been received for the previous laser pulse. Dual-axis-scan LiDAR units produce distance-measured points in two dimensions by using, for instance, a pair of polygonal mirrors. The horizontal scan mirror rotates at a faster rate than the vertical scan mirror.
Flash LiDAR devices like those disclosed in U.S. Pat. No. 8,072,581 offer a way to acquire a 3D map of a scene via a solid state or mostly solid state approach. These devices illuminate an entire 2D field of view with a blanket of light and measure the return value time for each photoreceptor location in the field of view. These approaches are relegated to very close proximity applications due to the low incident laser power for each location in the field of view. For flash LiDAR at longer ranges, the usable field of view is typically too small for applications like autonomous vehicle navigation without the use of high performance cameras operating in the picosecond range for exposure times.
U.S. Pat. No. 7,969,558 describes a LiDAR device that uses multiple lasers and a 360-degree scan to create a 360-degree 3D point cloud for use in vehicle navigation. The disclosed system has two limitations. First, the rotating scan head makes the unit impractical for widespread use on autonomous vehicles and makes it unusable for inclusion in mobile devices like smart phones, wearable devices, smart glasses, etc. Second, multiple units cannot work effectively in the same relative physical space due to the potential of crosstalk.
Scanning LiDAR units typically utilize a single laser, or multiple lasers, all operating at the same wavelength. Care must be taken to ensure that signals received by the photoreceptor are reflected light from the desired emitted source. Two LiDAR units, call them A and B, operating with lasers at the same wavelength have the potential to experience crosstalk. Inbound signals at the A detector wavelength of, for example, 650 nm could be a reflected signal from an emitter for unit A, a reflected signal from unit B, or a signal directly from an emitter of unit B. In an application like autonomous vehicle navigation with multiple LiDAR sensors per vehicle on a busy roadway, the potential for crosstalk among pulsed-laser LiDAR units is quite high.
Crosstalk interference between individual LiDAR units can be reduced by utilizing time division synchronization between the units wherein the transmit times of one unit do not overlap with the transmit times of other units. This synchronization of individual units will lower the capture rate for each device and is impractical when the individual units are integrated with separate, independently-controlled systems.
The error mode for crosstalk interference among LiDAR units will typically be one or more distances being computed as lower than the actual distances or failure to find a signal, resulting in no value being reported for an individual point. For LiDAR units that utilize signal intensity from the target information, the recording intensity will typically be higher than the actual intensity of the returned signal.
U.S. Pat. No. 8,363,511 attempts to overcome the crosstalk interference problem in short range object detection systems by emitting and detecting a series of encoded pulses as part of the ultrasonic or microwave waves generated by the transducers. While this kind of encoding technique has the potential to reduce some occurrences of crosstalk interference, encoding techniques are still not sufficient for applications that may encounter an unknown and large numbers of devices that are simultaneously operating at the same or similar wavelength of emitter energy.
U.S. Pat. No. 7,830,532 also attempts to address the crosstalk interference problem in the context of short range object detection systems using infrared light for fixed location units such as garage door sensor detectors by various combinations of time division, frequency division, encoding and testing modes. While these kinds of solutions might work in the context of limited numbers of fixed object detection systems, they are not practical or effective in the context of current LiDAR technologies, especially when used in moving environments.
LiDAR units have the potential to be utilized extensively in applications like autonomous vehicle navigation, mobile computing and wearable devices. However, problems remain in developing effective LiDAR units that can address the interference challenges and operate reliably in an environment where hundreds or thousands of like devices are operating simultaneously.